Heat-treatment of nickel and nickel containing alloys



United States Patent HEAT-TREATMENT OF NICKEL AND NICKEL CONTAININGALLOYS Eric Herbert Bncknall, Birmingham, England, assignor tls 'llieInternational Nickel Company, Inc., New York,

No Drawing. Application November 15, 1951, Serial No. 256,612

Claims priority, application Great Britain November 23, 1950 1 Claim.(Cl. 148-131) This invention relates broadly to the heat treatment ofmetals and alloys, and more specifically to such heat treatment in whichthe work is embedded in a metallic packing.

In heat-treating metals or alloys, it is generally required that thework shall not become heavily scaled, and in many cases it is alsostipulated that no appreciable change shall be produced in thecomposition of the metal or alloy layers underlying the surface. Forexample, in heat-treating steel it is a common practice to define theextent to which decarburisation of the surface is permissible or to whatdepth detectable decarburisation may occur. In addition to thislimitation of the undesirable elfects caused by oxidising or reducinggaseous atmospheres it is sometimes necessary to prohibit pick-up ofcontaminants such as sulphur during heat-treatment. This isparticularly, but not solely, the case when nickel or nickel-containingalloys (including nickel steels) are being heat-treated, since nickelhas a strong atfinity for sulphur and the presence of nickel sulphidecauses the important mechanical properties of these materials todeteriorate. Sulphur is commonly present to a dangerous extent in theatmospheres of furnaces fired with towns gas or oil and may also bepresent in electric furnaces in which packing media are employed orspecial atmospheres are introduced.

It is a common (but by no means uniformly successful) practice toheat-treat metals or alloys while embedded in a packing of metallicturnings or borings, normally of cast iron, within a substantiallyair-tight container. The packing tends to exclude the air or otherfurnace atmosphere from the work, and may act in a more positive way inpreventing surface attack, e. g., the high carbon content of cast irongives rise to a slightly carburising atmosphere which is an insuranceagainst decarburisation of steel. In common grey iron turnings andparticularly in malleable iron turnings there is, however, often enoughsulphur to cause deterioration of nickel steels and similar susceptiblematerials.

It is an object of this invention to provide an improved heat-treatmentprocess in which the danger of contamination of the work by sulphur ismaterially reduced.

It is another object of the invention to heat-treat metals or alloys ina special metallic packing.

It is a further object of the invention to heat-treat nickel andnickel-rich alloys in a metallic packing without causing the nickel ornickel-rich alloy to be contaminated by sulphur.

I have discovered that if the work to be heat-treated is embedded in ametallic packing containing a small proportion of magnesium as analloying ingredient improved results are obtained.

Magnesium is a metal with a high vapour pressure and an extremely highafiinity both for oxygen and sulphur. The high atlinity for sulphur hasthe consequence that a magnesium-containing alloy made by melting cannotcon- Patented Wi ey 24, 19 5573 tain more than a very small percentageof sulphur, because in the course of producing the alloy sulphur presentin the raw materials reacts with magnesium to form magnesium sulphide,which separates as a slag.

According to the invention, therefore, I use a magnesium-containingmetallic packing, and i find that this is a practical guarantee that thework will be protected from sulphur contamination as Well as fromscaling and decarburisation.

The magnesium content of the packing may be very low, and is preferablybelow 0.5%, but it should be at least 0.02%.

As an example, turnings from cast iron containing 0.1% magnesium as analloy constituent were used as a heat-treatment packing for a steelcontaining 8.5% nickel.

Comparison was then made of the mechanical properties of (l) a fullymachined test piece of steel treated in this way, (2) a similar testpiece of the same steel treated in turnings from ordinary grey iron and(3) an oversized test piece blank of the same steel treated in turningsfrom ordinary grey iron and subsequently machined to the same dimensionsas the other test pieces. In each case, the heat-treatment lasted for 1hour at 840 C., and at the end of this time the heat-treatment boxeswere removed from the furnace and the bars were extracted and quenchedimmediately in oil. All three bars were tempered at 200 C. for 30minutes in an oil bath. The mechanical properties were found to be:

It will be seen that the process according to the invention resulted ina combination of properties fully equal to that found in the interior ofan oversized blank, where no deterioration could have occurred due toattack during heat-treatment. The fully machined. specimen heattreatedin grey iron turnings had much inferior properties, a fact whichconfirmed that inadequate protection was provided by the ordinary greyiron turnings during heat-treatment.

The merits of the invention have also been shown by heating thin stripsof nickel, /20 nickel-chromium alloy, 80/ 15/5 nickel-chromium-ironalloy and 70/30 nickel-copper alloy in the same turnings. After 16 hoursat 1000 C., all four strips remained unembrittled and were littlediscoloured.

I claim:

In the process for heating under substantially nondecarburizingconditions an article made of a metal selected from the group consistingof nickel and nickelcontaining alloys susceptible to contamination bysulfur while said article is embedded in a packing of gray cast ironfragments, the improvement which comprises using as said packing graycast iron containing magnesium in amounts of about 0.02% to about 0.5%to prevent pickup of sulfur from the surroundings and to preventexcessive scaling and decarburization of said article.

References Cited in the file of this patent UNITED STATES PATENTS2,124,446 Detwiler July 19, 1938 FOREIGN PATENTS 260,646 Great BritainNov. 2, 1926 402,609 Great Britain Dec. 7, 1933

